High speed magnetic focus device

ABSTRACT

A focus coil for a cathode ray tube is capable of operating at high rates of change of flux with minimum losses and comprises a magnetically permeable, electrically non-conductive shell (exterior core) having an axial air gap substantially as long as the core, with a single helix of one turn per layer flat conductor, suitably insulated, as an electrical winding therein. Lack of eddy currents permits rapid changes in flux within the core, the large gap permits use of a small inductance and high current, and the high current is achieved by means of an extremely dense electrical winding formed of single-turn per layer flat conductor.

United States Patent 1 91 West 1 Jan. 16,1973

[ HIGH SPEED MAGNETIC FOCUS [57] ABSTRACT DEVICE 1 A focus coil for a cathode ray tube is capable of Inventor! Roger west, Weston, Connoperating at high rates of change of flux with 7 Assigneez United Ail-cm Corporation, East minimum losses and comprises a magnetically permea- Hartford, Conn.

Filed: NOV. 10, 1971 Appl. No.: 197,340

ble, electrically non-conductive shell (exterior core) having an axial air gap substantially as long as the core, with a single helix of one turn per layer flat con- I ductor, suitably insulated, as an electrical winding therein. Lack of eddy currents permits rapid changes [52] US. Cl. ..335/210, 313/84 in flux within the core, the large gap permits use of a [5i] Int. Cl ..H0lf 7/00 small inductance and high current and h high Cup [58] Field of Search ..335/2l0, 213; 313/84 rem is achieved by means of an extremely dense elec [56] References Cited trical winding formed of single-turn per layer flat conductor. UNITED STATES PATENTS 3,411,033 llll968 King ..335/210 x Primary Examiner-Ge0rge Harris 1 Chaim, 4 Drawing Figures Att0meyMelvin Pearson Williams i j 1' W 5 I ZZ HIGH SPEED MAGNETIC FOCUS DEVICE BACKGROUND OF THE INVENTION 1. Field of Invention This invention relates to cathode ray tube magnetic focus coils, and more particularly to a high speed dynamic focus device. v

2. Description of the Prior Art As is known in the art, focusing coils for cathode ray tubes typically comprise annular electromagnets comprising a hollow annular iron core having an internal annular gap displaced axially from the center of the core with a large number of turns of copper wire disposed within theannular iron core. The gap provides a high concentration of flux in substantially one position along the length of the neck of the CRT, providing a high axial magnetic field for focusing the electrons.

A recent innovation in cathode ray tubes is the multiple phosphor penetration tube which utilizes different phosphors for different colors, persistencies or other characteristics. The phosphors may be applied in layers separated by an electron barrier whereby the energy of the electron beam is selectively made only sufficient to reach a first layer of phosphor, or higher so as to penetrate the barrier and reach a second or additional layer of phosphor. In some modifications, the phosphors are arranged, instead, in homogeneous form with one phosphor completely surrounding the other, on a dot by dot basis.

Operation of multiple phosphor penetration type tubes require that the anode potential (the acceleration voltage) within the cathode ray tube be very quickly changed, at video rates, from one voltage (which may be on the order of 4 to 8 kilovolts) to a higher voltage (which may be on the order of 8 to 12 kilovolts). In order to achieve substantially the same brightness for each of the selected phosphors, it is necessary to alter the current due to the fact that brightness is a function of the product of the acceleration voltage and the current. Necessarily a lower acceleration voltage requires a higher current and vice versa. Changes in the acceleration voltage and beam current of the device completely alter the focusing requirements of the tube.

As is known, a magnetic lens is better than an electrostatic lens. For this reason, high quality cathode ray tube focusing systems employ magnetic focusing rather 7 than electrostatic focusing. In the case, however, of the rapidly in the core.

As is known in the art of magnetic lenses, the shorter the magnetic lens (that is, the shorter the length of field upon which the electrons are operated) the better the lens; that is, having less aberration, it permits focusing to the smallest spot. To achieve this, a small air gap is used. For example, for a l'rr inch diameter neck CRT, a gap of about one-fourth of an inch may be utilized so that substantially 90 percent of the flux does not even reach into the tube and only roughly percent does, providing an axially narrow field. In the steady state situation (that is, when focus is not changing very rapidly such as in a home television set), flux can be maintained very easily with DC current. In order to slew the focus of a penetration type tube known in the art, it may be necessary, typically, to change the focusing current by several amperes in a few microseconds, which would require voltages far in excess of that which can be handled by transistor circuitry. Despite the quality of the magnetic lens, electrostatic focusing has been used heretofore in multiple phosphor penetration type CRTs known to the art. However, characteristics of the penetration type tube further aggrevate the weaknesses of electrostatic focusing (electrostatic lenses). One characteristic of the electron optics of an electrostatic lens is defocusing which results from the mutual repulsion of the electrons in the beam. If there is a higher beam current, there is a greater density of electrons and so there is a greater repulsive force to aggrevate defocusing. Similarly, with a lower acceleration (or anode) voltage, the electrons travel at a lower speed so a given repulsive force will result in a greater dispersion of a commensurate increase in defocusing. In the case of the multiple penetration tube, both of these factors are present (low voltage-and high current) in the low penetration mode. Thus, electrostatic focusing used heretofore in penetration type tubes has left much to be desired.

SUMMARY OF INVENTION An object of the present invention is to provide improved cathode ray tube focusing means; a further ob ject is to provide cathode ray tube focusing means capable of being rapidly switched between two operating modes to accommodate widely differing cathode ray beam acceleration voltages; a further object of the present invention is to provide cathode ray tube focusing means capable of providing adequate focus to high voltage, low current electron beams.

According to the present invention, a cathode ray tube focus device comprises a magnetic. core of high magnetic permeability and low electrical conductivity having a maximum air gap adjacent the cathode ray tube, and employing a single helix (single turn per layerlof flat conductor within the core.

The present invention obviates the problems of high voltage and low current, permits rapid switching, and provides low inductance.

Other objects, features and advantages of the present invention will become more apparent in the light of the following detailed description of a preferred embodiment thereof, as illustrated in the accompanying drawing.

BRIEF DESCRIPTION OF THE DRAWING FIG. 1 is a simplified side elevation view of a cathode ray tube assembly;

FIG. 2 is a simplified sectioned, side elevation view of a magnetic focus coil known to the prior art;

FIG. 3 is a sectioned side elevation view of a magnetic focus coil in accordance with a preferred embodiment of the present invention; and

FIG. 4 is a perspective illustrating the shape of a core section in the embodiment of FIG. 3.

. 10 with a relatively snug trated in FIG. 2, comprises a ferromagnetic core structure 18 which is generally annular in shape and may be 3 UliSCRlP'l'ION OF THE PREFERRED liMllODlMIlN'l' Referring now td FIG. I, a cathode ray tube 10 known to the prior art is provided with a deflection coil 12 and a focus coil I4 which are usually generally annular in shape so as to slip over the neck I6 of the tube fit. The focus coil 14, as illusconsideredto comprise a pair of coaxial right cylinders 18a, 18b joined by circular end portions 18c, 18d. Within the core structure 18 is disposed a multi-turn coil 20, which is wound in an annular shape. Typically, the core l8 has an annular magnetic gap 22 within the cylindrical section 18a, which may typically be onefourth inch wide in a focus coil having a diameter of 3 to 4 inches and a length (axially) of 1% to 2 inches, for use with a tube having a neck 16 on the order of 1% inches in diameter. In such a deflection coil, there may be as much as 9 percent of the generated flux passing directly across the gap 22 without entering the neck 16 of the CRT, and only about 10 percent of the flux, illustrated by dotted lines 24, entering into the tube to form the magnetic field which comprises the magnetic lens for focusing the electron beam. This comprises a first obstacle to high frequency switching the flux in the magnetic lens: the inductance of the coil is many times greater than is necessary for the amount of flux actually working on the electron beam within the tube.

In prior art magnetic deflection coils 14, the core is typically made of a shell of soft iron, which provide a high magnetic permeability but is subject to eddy currents in the core when the flux changes. However, no matter how thin the shell is made, because the iron is an electrical conductor, there will be eddy currents to some degree. In the case of high frequency switching between a first level of flux and a second level of flux as described hereinbefore, the amount of eddy currents limit the rate at which the flux can be changed and further "increase the amount of power necessary to drivesuch a coil from one level of flux to another. In addition, the electric winding in the prior art focus coils 14 generally consists of a large number of turns, which results in a device having a high self inductance, and therefore the voltage required to drive the device, particularly where it is being operated between two different flux levels at a high rate of change is relatively high.

A focus coil 14a in accordance with the present invention, as illustrated in FIG. 3, comprises a ferrite core 30 which may be conveniently molded of sintered powered ferrite in two halves 30a, 30b, each of which (as illustrated in FIG. 4) may comprise a short right cylinder 32 having one end closed off by a circular shaped member 34, and having a suitable hole 36 to permit passage of the core 30 over the neck 16 of the tube. The ferrite may comprise any good quality powered ferrite and may preferably be one which provides a soft-loop hysteresis characteristic and low remanence. The utilization of a ferrite substantially eliminates any eddy currents and the losses attendant therewith, since the ferrite is an extremely poor electrical conductor. This allows an extremely rapid change of flux being carried in the core in contrast with iron cores 18 of the type known to the prior art.

Another aspect of the present invention is the utilization of a large (perhaps maximum) air gap in the vicinity of the neck 16 of the tube. As is illustrated in FIGS. 3 and 4, there is no interior cylindrical wall structure whatsoever in the present invention. This means that the air gap between one end (such as to the left in FIG. 3) and the other end (such as to the right in FIG. 3) is quite large in contrast with those known to the art. This means that substantially all of the flux within the core 30 must also pass through the tube 16. As a result, much less total flux has to be generated than is true in the case of the prior art where percent of it is wasted. This permits lowering of the inductance of the coil.

A third aspect of the invention is the provision of but a single helix of flat conductive material 40 which provides an electrical winding having but a single turn per layer. This allows a very high current density per unit of space within the core 30, and an extremely low resistance in the electrical winding, which together further reduce the losses in the coil. The electric winding 40 may comprise a single sheet of copper in the neighborhood of 1% inches wide and several mils thick. Each turn is insulated from adjacent turns by a suitable transformer insulation which may, for instance, take the form of two mil kraft paper 42 or the equivalent. As the paper and copper layers are wound together, they may be cemented with a suitable epoxy, and when the electrical winding is complete, it may be baked so as to form a relatively hard, integral unit, which may then be inserted between the two core parts 30a, 30b. Connection to the copper 40 may be made in any suitable fashion such as bending over each end to form an ear to which a suitable lead wire may be soldered or otherwise bonded for electrical conduction. Such connection leads as are used may conveniently be brought out through a pair of notches 44, 46 formed in one half 30b of the core 30. Of course other suitable electrical connections may be made as desired to suit any implementation of the present invention. Thus, the present invention provides a magnetic circuit which performs adequately at high rates of change of flux, due to the utilization of an all-ferrite core 30. The invention limits the self induced voltage in the focus coil to a magnitude suitable for control by transistor circuitry by lowering the inductance significantly and utilizing a high current instead. The inductance is lowered by utilizing all the flux which is generated due to the extremely large air gap, and the current is increased, without increasing the resistive losses, the electric winding or the size and weight of the device, by means of very compact single turn per layer conductor.

Although the invention has been shown and described with respect to a preferred embodiment, it should be understood by those skilled in the art that various changes and omissions in the form and detail thereof may be made therein without departing from the spirit and the scope of the invention.

Having thus described a typical embodiment of my invention, that which I claim as new and desire to secure by Letters Patent of the United States is:

1 A low inductance cathode ray tube magnetic focus coil adapted for use with high current and operable at high rates of change of flux, comprising:

a hollow, generally annular ferrite core of high magpieces to another of said circular pieces; and

netic permeability and low electrical conductivity An l trical winding comprising a single helix of flat and having an axial passageway therethrough adapted for insertion over the neck of a cathode ray tube, said core being in the form of a right cylinder having circular end pieces with axial holes formed substantially centrally in each of said circular end pieces, said core having an air gap extending substantially from one of said circular axial holes. 

1. A low inductance cathode ray tube magnetic focus coil adapted for use with high current and operable at high rates of change of flux, comprising: a hollow, generally annular ferrite core of high magnetic permeability and low electrical conductivity and having an axial passageway therethrough adapted for insertion over the neck of a cathode ray tube, said core being in the form of a right cylinder having circular end pieces with axial holes formed substantially centrally in each of said circular end pieces, said core having an air gap extending substantially from one of said circular pieces to another of said circular pieces; and An electrical winding comprising a single helix of flat conductive material formed into a single turn per layer winding each turn insulated from each adjacent turn, the innermost turn of said winding being of substantially the same diameter as said axial holes. 